1. Field of the Invention
This application relates to UV curable silicone compositions, which demonstrate high resistance to flammability and combustibility, and to silicone (polyorganosiloxane) rubber materials produced therefrom.
2. Brief Description of Related Technology
Silicone rubber and liquid compositions exist in various forms as characterized by their differing cure chemistry, viscosity, polymer type and purity. They can be formulated into one-part or two-part systems, and a particular silicone composition can be engineered to be curable by more than one mechanism.
Moisture-curing mechanisms, heat-curing mechanisms, and photo-initiated curing mechanisms are among the means used to initiate cure, i.e., cross-linking of reactive silicones. These mechanisms are based on either condensation reactions, whereby moisture hydrolyzes certain groups on the silicone backbone, or addition reactions that can be initiated by a form of energy, such as electromagnetic radiation or heat. In certain silicone compositions, a combination of such cure mechanisms may be used to achieve the desired results.
For example, reactive polyorganosiloxanes can be cured by heat in the presence of a peroxide. Alternatively, these reactive siloxanes can also be cured by heat in the presence of silicone hydride-containing (--SiH) compounds and a metallic hydrosilylation catalyst, such as an organo-platinum catalyst.
UV curing silicones having methacrylate functional groups are known. For instance, U.S. Pat. No. 4,675,346 (Lin) is directed to UV curable silicone compositions including at least 50% of a specific type of silicone resin, at least 10% of a fumed silica filler and a photoinitiator, and cured compositions thereof.
Other known UV curing silicone compositions include those disclosed in Great Britain Patent No. 1,323,869 [composition for plate-making in printing consisting of an organopolysiloxane containing a (meth) acrylate functional group, a photosensitizer, and a solvent, which cures to a hard film]; U.S. Pat. Nos. 4,201,808 (Culley) and 4,348,454 (Eckberg) (compositions of an organopolysiloxane having an average of at least one acryloxy and/or methacryloxy group per molecule; a low molecular weight polyacrylyl crosslinking agent; and a photosensitizer, reported as being suitable for release coatings and curable upon exposure to UV radiation); and Great Britain Patent No. 2,039,287 (composition for protective coatings on paper prepared from the reaction of methacryloxypropyltrimethoxysilane and low molecular weight hydroxyl-terminated polyorganosiloxanes).
In certain commercial applications of such silicone formulations, flammability and combustibility is a concern, particularly where it is an issue in production or with the end user. As a result, ordinarily only a subset of such silicone formulations have been used in such instances.
In the past, to alleviate the concern, heat-curable silicone compositions have been used where moldability of the composition itself and electrical properties of the cured product were desirable. Such heat-curable silicone compositions have been rendered resistant to flammability and combustion through the addition of halogenated materials, fillers, organo-platinum materials, and benzotriazoles.
However, the use of halogenated materials to achieve that property is undesirable because of toxicity and environmental concerns, as well as possibly contaminating the substrate on which the composition is placed.
Other ways of rendering such heat-curable silicone compositions are also known. For instance, European Patent Application Nos. 801 111, 808 874 and 808 875, each relate to liquid silicone rubber compositions which, after cure, are reported as producing a highly flame retardant silicone rubber. The compositions are polyorganosiloxane-based, and contain silica filler, aluminum hydroxide, benzotriazole, and a platinum-3,5-dimethyl-1-hexyn-3-ol reaction mixture. The compositions may further include zinc carbonate, polyorganohydrogensiloxane, and a hydrosilylation reaction catalyst.
In addition, a recently-published Chemical Abstracts citation to a Chinese patent document refers to a flame retardant silicone rubber for dielectric coatings of electric wire containing a vinyl-siloxane rubber, benzotriazole, chloroplatinic, a silica filler, and hydroxy silicone oil. See Chem. Abs. 128:168854s (1998)(citing Chinese Patent Document CN 1,144,237). Vinyl-siloxane rubbers are ordinarily curable by thermal mechanisms.
However, with certain commercial applications, heat-curable silicones present shortcomings. Those shortcomings include their limited usefulness with heat-sensitive substrates and/or heat-sensitive electronic circuitry. In addition, heat-curable silicones generally are slower to cure than silicones curable through other mechanisms, such as uv cure. Such a shortcoming is seen to be troublesome in the fabrication of devices, where the slower cure speed renders the fabrication process otherwise less efficient. Therefore, the applicability of such heat-curable silicones across a wide range of end-use applications appears to be limited.
Indeed, in those applications where a UV curable silicone composition would be desirable, one could not simply use the flame retardant materials as they have been used in the past with heat-curable silicones to impart such properties. That is, certain of these filler materials generally (and in the amounts called for by the EP '111, '874 and '875 publications) would render the compositions less transmissive to light (and oftentimes opaque). Compositions containing such fillers, thus, may not be cured properly by exposure to UV radiation. That is, at best a surface skin may form, but cure through volume likely will not occur due to interference by the filler.
Moreover, in practice, the compositions themselves would exhibit poor flow properties because of the amount of fillers used. And, when cured, the compositions may be brittle and therefore reduce the integrity of bonds formed therefrom.
And, a recently-published Chemical Abstracts citation to a Japanese patent document refers to UV-curable polysiloxane coating compositions reportedly having flame retardant capabilities. These silicone compositions contain a siloxane component, 2-hydroxy-2-methyl-1-phenylpropane and aluminum acetylacetonate. These compositions do not however appear to be filled, and there therefore may not possess the strength required of a cured material in a sealant or conformal coating application.
Accordingly, a need is seen for a UV curable silicone which is also resistant to flammability and combustibility.
Despite the existence of this need, it is not believed to date that UV curable filled silicone compositions have been made flame- and combustion-retardant.
Thus, notwithstanding the state-of-the-silicone technology with respect to flame and combustion retardancy, it would be desirable for a flame-retardant silicone compositions to be curable by exposure to UV radiation.